In recent years, gamma ray spectroscopy of earth formations adjacent to a well borehole has been enhanced by highly stable scintillation detectors. Such detectors are normally mounted in a sonde which is lowered in a well borehole. They produce a pulse height spectrum proportion to the gamma ray energy spectrum of the gamma rays impinging on the scintillation crystal. The gain stabilized scintillation detector of the present invention utilizes the naturally occurring gamma radiation emitted from earth formations. The gamma radiation is produced primarily by potassium, uranium and thorium (and/or their daughter decay products).
Naturally occurring gamma radiation is emitted isotropically from unstable nuclides in earth formations whereupon it is transmitted through the formation and borehole materials to impinge on a scintillation crystal in the well borehole. Photoelectric absorption, Compton scattering, and pair production attenuate the radiation between its source generation in the formation and its detection in the crystal. The crust of the earth is a layered medium composed, in large part, of silicon, hydrogen, carbon, calcium, chlorine, oxygen and other relatively low atomic weight elements. The same elements are principally present in cement, mud, and borehole water or oil between the formation and detector. The iron in well casing has a higher atomic number than virtually all of these other elements, and also attenuates the naturally occurring gamma radiation prior to its being detected. However, significant photoelectric absorption in iron takes place at a higher energy level than in these other elements, and hence one can effectively isolate the attenuation due to iron from that due to other elements by using an energy dependent measurement of this photoelectric attenuation. This data, if properly normalized, can provide a measurement of the amount of iron (and hence casing thickness) which is present in a well.
Virtually all producing wells are cased. At the time that a well is cased, the casing program may be readily available whereby nominal casing thickness can be determined. However, many wells, due to changes in formation pressures as a function of depth, require several strings of different size casing to be incorporated into the drilling program. In drilling out below a depth to which casing has been set, it is possible that the casing may wear due to contact with the drill string used to drill out the deeper section in the well. This wear results in a decrease in casing thickness, which in turn weakens the casing. In many other instances, casing of a known thickness was installed in a well. However, after years of exposure to external deterioration caused by formation fluids, and internal deterioration due to produced fluids, corrosion may very well reduce the thickness, and hence strength, of the casing. In order to avoid casing collapse or blowouts, it is necessary to know the degree of wear or corrosion which has taken place, and to take remedial action if sufficient reduction in casing thickness has taken place. For these reasons, it is extremely valuable to be able to determine the thickness of the casing.
Photoelectric absorption is a gamma ray attenuation process whereby the gamma ray is completely removed in the interaction. The cross section, or probability, of photoelectric absorption (.sigma.) is strongly affected by the energy E of the gamma ray and the atomic number, Z, of the element with which it interacts. This relationship is approximated by: EQU .sigma..about.Z.sup.4.6 /E.sup.3 ( 1)
Since iron has a higher Z (=26) than most earth formation elements, below a given energy level (about 175 KeV), iron dominates this absorption process. Above about 200 KeV, photoelectric absorption in iron, as well as other principal downhole elements is not significant. Hence if iron is present in the formation or borehole in vicinity of the logging tool detector, then the count rate in the detector in the tool at energies below about 175 KeV is decreased relative to the count rate in a higher energy range in which range photoelectric absorption is not significant. In addition, since iron has a higher Z than other elements in cement and borehole fluids, this relative attenuation increase in the lower energy range is virtually independent of changes in borehole geometry and composition. Changes in these variables will similarly influence count rates both below and above 175 KeV, as will changes in the density and lithology type of the formations from which the gamma rays are originally emitted. Of course, the concentration of radioactive source materials in the formation will also be similarly reflected both above and below 175 KeV.
Since casing thickness changes produce a pronounced relative count rate change (more than the other variations described above) in the very low energy region of the detected natural gamma spectrum, it is possible to isolate the effects of photoelectric absorption in iron. A ratio comparison of the summed counts in two energy ranges, one of which is sensitive to photoelectric absorption in iron, and (a higher) one in which photoelectric absorption in iron is negligible, is sensitive to changes in iron thickness, but not to changes in other downhole parameters. This ratio, when compensated for the geometrical differences between casings of different sizes, can be used to directly indicate the thickness of the casing in the borehole. The above described energy range measurements require a highly stable scintillation detection system. In the present invention a coincidence technique is used to gain stabilize the detector to a precise degree.